Head cracking is one of the most frustrating failure modes for screws. The screw looks fine in the box, but as soon as the installer applies torque, a crack appears around the head or the head suddenly breaks off. This leads to warranty claims, project delays, and a loss of confidence in both the fastener and the supplier.
For overseas purchasers, quality engineers, and distributors, head cracking is not only a technical problem but also a communication and risk-control issue. Understanding why screw heads crack and how to prevent it will help you choose the right suppliers, write clearer specifications, and avoid expensive rework on site.
In this guide we will look at the main causes of head cracking in screws, from raw material and cold heading to heat treatment, plating, and installation. Then we will translate these technical points into a practical purchasing and inspection checklist you can use in real projects.
1. What Is Screw Head Cracking?
Head cracking usually refers to visible cracks that start at the fillet between the screw head and shank, or around the recess, and grow under torque or load until the head breaks. This is different from simple drive-bit cam-out or stripped recess; the metal itself is cracking.
Head cracking can appear:
- During installation – the head splits when the screw is near its final torque
- Shortly after installation – delayed cracking due to internal stresses and hydrogen embrittlement
- In storage – visible radial cracks around the head on unused screws, often related to severe manufacturing defects
In many cases the root cause is a combination of high hardness, stress concentration in the head design, and internal defects or hydrogen in the steel. Once you know where these risks come from, you can discuss preventive actions with your supplier more effectively.
2. Production-Related Causes of Head Cracking
2.1 Raw material quality
Head cracking problems often begin with the wire rod used for cold forming. Poor-quality wire can contain:
- Longitudinal seams and laps
- Large non-metallic inclusions
- Segregation or banding in the microstructure
These defects become stress concentrators in the highly formed head area. When the screw is hardened and then loaded in service, cracks initiate along these weak zones.
For heat-treated tapping screws, standards such as ISO 2702:2022 specify mechanical and physical properties that help prevent brittle failures and ensure screws can form mating threads without breaking.
From a purchasing point of view, you can ask the supplier:
- Which steel grade is used for your screws?
- Do you have material certificates from the steel mill?
- What incoming inspection do you perform on wire (surface, diameter, hardness)?
Even if you do not review every technical detail, asking these questions signals that you care about head cracking risk and expect traceable materials.
2.2 Cold heading and head geometry
Cold heading is where the screw head shape is formed. Key risk points include:
- Insufficient fillet radius between the head and shank
- Too sharp transitions or under-filled areas in multi-blow forming
- Off-centre head creating uneven stress distribution
- Thin head height or reduced cross-section near the recess
If tooling is worn or incorrectly designed, small laps and folds may appear around the head or fillet area. These become ideal starting points for cracks under torque.
Buyers cannot see the tooling directly, but you can:
- Request sample screws and examine the head–shank fillet under magnification
- Ask whether the supplier performs regular head soundness checks or metallographic inspection on cross-sections
- Avoid pushing for extreme cost reductions that force the use of marginal tooling or wire quality
2.3 Heat treatment issues
Many self-tapping and self-drilling screws must be hardened to achieve the required strength and drilling performance. However, if the screw is too hard and not tough enough, the head will be brittle.
Problems can arise from:
- Over-hardening or insufficient tempering
- Non-uniform temperature in the furnace causing hardness variation
- Quench cracks originating at the fillet or under the head
- Decarburization and re-carburization zones that create weak or hard layers
ISO 2702 describes mechanical properties and test methods for heat-treated tapping screws to make sure they can perform without deforming their own threads or breaking during assembly.
When you discuss heat treatment with your supplier, focus on:
- Hardness range (for example, by Rockwell C or Vickers) suitable for your application
- Whether they use continuous furnaces with controlled atmosphere
- How often they check hardness and microstructure across the head and shank
The goal is not to make the screw as hard as possible, but to strike a balance between strength, ductility, and resistance to cracking.
2.4 Plating and hydrogen embrittlement
Electroplating processes such as zinc or zinc-nickel plating can introduce hydrogen into high-strength steel. If the hydrogen is not removed by baking, it may cause hydrogen embrittlement, leading to delayed cracking of the head or shank under stress. Technical papers on hydrogen embrittlement in fasteners show that three factors are needed for this failure mode: high tensile strength, tensile stress, and absorbed hydrogen.boltcouncil.org+2fastenal.com+2
Typical risk factors include:
- Acid pickling before plating
- Poor control of plating bath parameters
- No or insufficient post-plating baking
- Very high hardness levels (for example above around 39–40 HRC in some guidelines)
If you have recurring head cracking issues on plated screws but not on unplated versions, hydrogen embrittlement is a strong suspect.
To reduce this risk, specify:
- Maximum hardness suitable for the application
- Post-plating baking requirements (temperature and duration) for critical screws
- Avoid unnecessary electroplating on very high-strength small screws when other coatings (e.g., mechanical plating, zinc flake) are acceptable for corrosion performance
3. Standards and Design Considerations
3.1 Relevant screw standards
Different screw types are covered by different standards. For example:
- ISO 2702 – Heat-treated steel tapping screws; defines mechanical and physical properties for tapping screws made of steel.
- AS 3566 – Self-drilling screws for the building and construction industries; specifies dimensions, thread forms, mechanical properties, corrosion resistance, and performance requirements.codehub.building.govt.nz+1
These standards include torsional strength, drive testing, and sometimes head bursting or pull-through requirements which are directly related to head cracking resistance.
When you write your RFQ or purchase order, it is better to reference the correct standard (e.g. “self-drilling screws to AS 3566.1, Class 3 coating”) instead of only giving size and head type. This ensures that both you and the supplier share the same expectations for mechanical performance.
3.2 Head design and recess choice
The geometry of the head and recess has a strong influence on cracking risk:
- Fillet radius between head and shank should be generous enough to reduce stress concentration.
- Head height must be adequate for the drive type; very thin heads are more likely to crack or strip.
- Recess size and depth need to match standard driver bits; undersize recesses concentrate force.
- Drive type (Phillips, Pozi, Torx, hex, etc.) changes torque distribution and cam-out behaviour.
Torx and other internal drives with large contact area generally allow higher torque with less risk of cam-out compared with traditional Phillips drives. For high-torque applications such as timber construction or deck screws, this can be an advantage.
When you develop customised screws with a supplier like Linkworld, you can discuss whether your current head shape and drive are optimal for the expected installation torque and substrate.
4. Installation-Related Causes of Head Cracking
Even perfectly manufactured screws can crack if they are installed incorrectly or used in the wrong application. Buyers and distributors should share basic installation guidelines with customers to reduce misuse.
4.1 Over-torque and wrong tools
Common on-site causes of head cracking include:
- Using impact drivers or powerful cordless drills without torque control
- Using the wrong bit size, which concentrates force on a small area of the recess
- Driving the screw long after the head has seated, trying to “squeeze” parts together
- Using worn bits that slip and strike the head edges
Training installers to stop once the screw is properly seated, and to choose the correct bit and torque setting, will reduce both head cracking and recess stripping.
4.2 Misalignment and bending
If the screw is not perpendicular to the work surface, it can experience bending loads under the head. Bending combined with high torque can produce cracks at the fillet even when the material itself is within specification.
Typical situations include:
- Screws driven at an angle into steel purlins
- Screws forced through misaligned holes or pre-punched sheets
- Long screws spanning gaps without proper support
Encouraging the use of guiding tools, pre-drilled pilot holes where appropriate, and good fixture alignment helps reduce these issues.
4.3 Substrate hardness and pre-drilling
Using self-tapping screws in steel that is too hard or too thick for their design can cause excessive torque and head cracking. Standards such as AS 3566 specify the range of steel thickness and hardness for which self-drilling screws are suitable.codehub.building.govt.nz+1
If the substrate is outside that range, pre-drilling or a different fastener type (e.g. concrete screws or masonry bolts) may be needed. For example, for heavy attachments to concrete or brick, a product such as the Slotted Concrete Screw or Masonry Bolts / Screw Bolts from Linkworld may be more appropriate than a standard self-drilling screw:
https://linkworldfast.com/product/slotted-concrete-screw/
https://linkworldfast.com/product/masonry-bolts-screw-bolts/
5. Purchasing Specifications to Reduce Head Cracking Risk
From a buyer’s perspective, you cannot control every operation in the factory or on the job site, but you can control what you specify and what you check.
5.1 Define the correct standard and application
For each screw in your project, specify:
- Whether it is a self-tapping, self-drilling, wood, or concrete screw
- Applicable standards (for example ISO 2702 for tapping screws, AS 3566.1 for self-drilling roofing screws)
- Intended substrate (timber, steel thickness, concrete, etc.)
- Expected installation method (manual, cordless driver, impact tool, etc.)
For construction and roofing applications, screws that comply with AS 3566 offer defined performance levels for drilling, torsion, and corrosion resistance.
On the Linkworld website you can find several construction screw options, such as AS3566 Bugle Head Batten Screws and other roofing screws in the screws category:
https://linkworldfast.com/product/as3566-bugle-head-batten-screw/
https://linkworldfast.com/product-category/screws/
5.2 Specify hardness and coating correctly
Instead of simply saying “case-hardened screw with zinc plating”, try to include:
- Target hardness range (for example, core hardness in HRC or HV values suitable for your codes)
- Case depth requirements if relevant for self-drilling screws
- Coating type (electro-zinc, mechanical plating, Ruspert, hot-dip galvanizing, etc.)
- Hydrogen relief baking requirement for electroplated high-strength screws
For outdoor timber applications, for example, T17 Ruspert Wood Screws offer corrosion-resistant coating and a T17 point for easy penetration:
https://linkworldfast.com/product/t17-ruspert-wood-screws/
Clear specifications make it easier for the supplier’s quality team to design a process window that balances hardness, toughness, and corrosion resistance.
5.3 Ask for key process and test information
You do not need to audit every furnace, but you can request some basic documentation and test results, such as:
- Description of heat treatment and tempering process used for the screws
- Hardness test reports showing values in the head and shank
- Torsion or drive tests demonstrating that screws can reach a certain torque without head cracking
- Plating line description and hydrogen relief baking procedure for high-strength, electroplated screws
If a supplier is familiar with ISO 2702 or AS 3566 testing, they can usually provide relevant data or do additional tests when required.
6. Incoming Inspection and Simple Field Tests
Even with good specifications, you still need to verify that delivered screws meet your expectations.
6.1 Visual inspection
On arrival, perform basic checks on a reasonable sample:
- Look for radial cracks around the head or fillet area under good lighting
- Inspect the recess for burrs, incomplete forming, or plating build-up
- Check that head markings (if any) and plating appearance are consistent lot-to-lot
- Compare head shape and dimensions with your drawings or approved samples
If you already see head cracks or severe forming defects before installation, quarantine the batch and contact the supplier immediately.
6.2 Simple torque and drive tests
In addition to formal laboratory tests, simple shop-floor checks can reveal potential head cracking risks:
- Install screws into the intended substrate using the same tools as on site
- Gradually increase torque until the screw is properly seated, then add a safety margin
- Observe whether the drive strips, the thread fails, or the head cracks
Record the approximate torque level or tool setting at which problems occur. If head cracking appears at relatively low torque compared with your expectations, request a detailed investigation from the supplier.
6.3 When you find a problem
If you experience head cracking in the field:
- Collect evidence – cracked samples, installation tools, torque settings, photos of the joint.
- Separate lots – do not mix potentially problematic screws with other deliveries.
- Share information – send details to the supplier, including batch numbers, installation conditions, and whether the screws were plated or unplated.
- Ask for root cause analysis – for example, an 8D report including metallographic examination of cracked and uncracked screws.
A cooperative supplier will look at material, heading, heat treatment, plating, and installation data to identify the true root cause and propose corrective actions.
7. Example Case: Head Cracking in Roofing Screws
Consider a roofing project using self-drilling screws to AS 3566. After a few weeks on site, installers report that some screw heads crack when tightening the final few turns. Not all screws are affected, but enough to slow down work and raise concerns.
A joint investigation between the buyer and the supplier shows:
- Screws meet hardness requirements, but are at the high end of the range.
- Plating is electro-zinc without sufficient baking time.
- Installers are using powerful impact drivers with long extension bars.
- Some roof sheets are thicker than originally specified, increasing drilling torque.
Corrective actions include:
- Slightly lowering the maximum hardness in heat treatment.
- Increasing post-plating baking time for hydrogen relief.
- Updating installation instructions to limit impact driver settings and use correct driver bits.
- Considering a higher-performance screw design from the same supplier for the thicker sheets.
After these changes, head cracking complaints drop dramatically.
8. Working With Linkworld to Control Head Cracking
Jiaxing Linkworld Fastener combines in-house cold forming and machining with cooperative factories for related parts, allowing integrated production and quality control for a wide range of screws and other fasteners. This setup supports both standard screws and customised designs based on your drawings.
For screw applications where head cracking is a concern, Linkworld can:
- Discuss suitable steel grades and hardness ranges for your products
- Optimise head geometry and recess type for the expected torque and substrate
- Coordinate heat treatment, plating, and baking processes to reduce embrittlement risk
- Provide test data such as torsion tests, drive tests, and hardness results for key projects
- Supply related products—such as bolts, concrete screws, and rigging parts—in the same shipment for easier logistics
You can explore the general product portfolio here:
https://linkworldfast.com/products/
For screw-specific solutions, including deck screws, roofing screws, and woodworking screws, start with the screws category:
https://linkworldfast.com/product-category/screws/
To learn more about the company background and manufacturing capabilities, visit:
https://linkworldfast.com/about-us/
9. Summary and Next Steps
Head cracking in screws is a multi-factor problem. It can be driven by poor raw material, aggressive head forming, over-hard heat treatment, hydrogen embrittlement from plating, or simply over-torque and misalignment during installation. As a purchaser or quality engineer, you cannot control every detail, but you can build a framework that makes head cracking much less likely:
- Choose the correct standard and screw type for each application.
- Specify reasonable hardness ranges and appropriate coatings, including baking when needed.
- Ask suppliers to share key process controls and test results related to head integrity.
- Perform basic visual and torque tests on incoming batches.
- Work closely with your supplier and installers to correct any problems quickly.
If you would like to discuss head cracking issues in your current screws, optimise specifications for a new project, or develop customised screws based on your drawings, you are very welcome to contact the Linkworld team. You can visit the homepage at
https://linkworldfast.com/
browse the product range at
https://linkworldfast.com/products/
or send an inquiry via
https://linkworldfast.com/contact/
or by email at info@linkworldfast.com.